1. Field
One embodiment of the invention relates to a recordable and/or erasable information recording medium. This embodiment particularly relates to an optical recording medium having one or more recording films configured to record information by using a phase change. More specifically, the embodiment relates to a single-sided, multi-layer phase change optical disc suitable for repetitive recording/playback of large-volume digital data such as high-resolution video information.
Or, one embodiment of the invention relates to an optical recording medium which records information by reversibly changing a state by emitting a light beam. This embodiment particularly relates to a phase change optical recording medium in which the atomic arrangement of a thin film for holding information changes between an amorphous state and crystalline state, thereby causing a state change for information recording.
2. Description of the Related Art
(Phase Change Optical Recording Principle)
In a general phase change optical recording film, a portion heated to a melting point or more melts, and this melted portion takes an amorphous atomic arrangement when rapidly cooled. Also, when the film is held in the temperature region of a crystallization temperature which is below the melting point for a predetermined time or more, the film remains in a crystalline state if it is initially crystal, but crystallizes if it is initially amorphous (a solid phase erase mode). Depending on the material of the recording film, it is also possible to crystallize the recording film by melting a portion near a non-crystal portion of the recording film by heating the portion to a melting point or more, and slowly cooling the melted portion (a melt erase mode).
Since the intensity of reflected light from an amorphous portion and that of reflected light from a crystal portion are different, the intensity of reflected light is converted into that of an electrical signal, and information is read out by A/D-converting the electrical signal. This is the principle of the phase change optical recording medium. It is also possible to record and read out information by using a change between a metastable crystal phase such as a martensite phase and a stable crystal phase, or a change between a plurality of metastable crystal phases, instead of the crystal-amorphous phase change.
(Method of Increasing Density)
The following two methods can be used to increase the amount of information recordable on one recording medium, i.e., increase the recording capacity. One is a method which reduces the pitch of recording marks in a track direction. If the degree of downsizing advances, however, the pitch size reaches a region smaller than the size of a playback light beam, so a playback beam spot may temporarily include two recording marks. If the recording marks are well separated from each other, a playback signal is largely modulated, and a signal having a large amplitude is obtained. However, if the recording marks are close to each other, a signal having a small amplitude is obtained, so an error easily occurs when the signal is converted into digital data.
The other method of increasing the recording density is to reduce the track pitch. This method can increase the recording density without being largely influenced by the reduction in signal intensity caused by downsizing of the mark pitch described above. Unfortunately, this method has the problem that in a region where the track pitch is equal to or smaller than the size of a light beam, information on a certain track deteriorates while write or erase is performed on an adjacent track. That is, so-called cross erase occurs.
The causes of the cross erase are that a mark is directly irradiated with the outer edge of a laser beam on an adjacent track, and that a hot stream during recording flows into an adjacent track and raises the temperature of a mark on the adjacent track, thereby deteriorating the shape of the mark. These problems have to be solved to increase the density of the phase change optical recording medium. Also, to accurately read a downsized mark and decrease the probability of a read error, it is desirable to minimize the noise component by smoothing the shape of the outer edge of a recording mark to be formed.
(Method of Increasing Capacity by Multi-Layer Medium)
Another method of increasing the capacity is to stack a plurality of layers for holding information. This method is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-322770 (a dual-layer RAM system). A medium having two stacked layers and designed to be readable and writable from one side is called a single-sided, dual-layer medium or simply called a dual-layer medium. In this single-sided, dual-layer medium, an information layer (to be referred to as L0 hereinafter) formed close to the light incident side has to ensure a transmittance of approximately 50[%] or more so as not to attenuate light excessively in L0 when an information layer (to be referred to as L1 hereinafter) formed far from the light incident side is accessed. To this end, the thickness of the recording film has to be as very small as 10 nm or less in L0.
Decreasing the film thickness prolongs the holding time for crystallization, and produces unerased information at a normal recording speed. “Manuscripts for The 12th Phase Change Recording Research Meeting Symposium (Proceedings of PCOS2000) pp. 36-41” discloses that a method of substituting a portion of a GeSbTe recording film with Sn is effective as a measure to counter this problem. Also, Jpn. Pat. Appln. KOKAI Publication No. 2001-232941 (a GeSbTeBi series) discloses that substituting a portion of a GeSbTe recording film with Bi, In, Sn, or Pb is similarly effective. To ensure the erase ratio described above, however, improvements of the recording film material are unsatisfactory, and a film having a crystallization promoting effect is formed in the interface with the recording film.
According to “Manuscripts for The 12th Phase Change Recording Research Meeting Symposium”, germanium nitride (GeN) is effective. However, the inventors made extensive studies, and found that when a very thin film made of the recording film material described above and having a thickness of 10 nm or less is combined with the conventional interface film material such as GeN, the cross erase described previously occurred, so the track pitch could not be decreased. Also, when silicon carbide (SiC) reportedly having a crystallization promoting function is used, the extinction coefficient of light is large at a wavelength of 405 nm of a laser beam used for the next-generation, high-density optical disc, and this produced a very large optical loss. In addition, germanium nitride (GeN) or silicon nitride (SiNx) also produced an optical loss.
On the other hand, in a medium having no interface film, it is possible to suppress recrystallization of the melted portion and reduce the cross erase, but the erase ratio is totally unsatisfactory. Furthermore, recording and erase have to be performed on L1 by using a laser beam attenuated to approximately a half beam by passing through L0. Therefore, it is very important to decrease the optical loss in an interface layer or dielectric layer, in order to meet the requirements for a high density of the medium and increase the utilization ratio of the emitted laser beam at the same time.
(Method of High-Speed Recording)
High-speed recording is another requirement for phase change optical recording. When an image is to be recorded, for example, if the image can be recorded within a time shorter than the actual playback time, it is readily possible to realize a so-called time shift function (time slip playback) by which a user can view previous images by tracing back the time during dubbing of a distributed medium or recording of broadcast. One main cause which prevents high-speed recording in phase change recording is the problem that information is kept unerased when crystallization is performed by a laser having a relatively low erase level during overwrite, i.e., the problem of an insufficient erase ratio. This problem arises because a recording mark rapidly passes through a laser spot and hence does not stay in a temperature region capable of crystallization for a sufficiently long time, so information is kept unerased.
Jpn. Pat. Appln. KOKAI Publication No. 11-213446 discloses a method which raises the erase speed by promoting crystallization by forming a material such as GeN in the interface with a recording film. However, when the inventors conducted experiments by using the material disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-213446 as an interface film, a part of a portion melted during recording recrystallized, i.e., to form a recording mark having a necessary size, a range larger than the recording mark has to be melted. The use of an interface film like this melts an unwanted large region, and consequently accelerates the cross erase. Accordingly, this method has an adverse effect from the viewpoint of high-density recording.
In other words, when the material disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-213446 is used as an interface film and information is recorded by a laser power within a range which is allowable in respect of the cross erase, the width of the formed recording mark decreases, and this lowers the obtained signal-to-noise ratio (or carrier-to-noise ratio: CNR). On the other hand, in a medium having no interface film, it is possible to suppress recrystallization of the melted portion and reduce the cross erase, but the erase ratio is totally unsatisfactory. That is, a new interface film material capable of raising the crystallization rate during erase (ensuring a necessary erase ratio even in high-speed recording) and also capable of suppressing recrystallization of the melted portion during recording (reducing the cross erase) is desired.
(Film Design of Phase Change Optical Recording Medium)
As explained in “phase change optical recording principle”, in the phase change optical recording medium, an amorphous mark is formed, i.e., data is written in a desired portion of a recording film by emission of a laser pulse, and data is erased by crystallization by irradiating an amorphous mark with a low-power laser. In the data write, an amorphous mark is formed by rapidly cooling a portion irradiated with the laser. In the data erase, an amorphous portion is crystallized by slow cooling. If the recording film has a high laser absorbance, operations such as recording and erase can be performed with a low laser power. If this absorbance is low, a high laser power is needed for recording and erase. This absorbance in the recording film is determined by the optical characteristics and thermal characteristics of the individual film materials of the medium formed by a multi-layer film. For example, the arrangement can be changed by selecting film materials equal in absorbance, and it is possible to produce anisotropy in thermo-physical properties between a rapid-cooling structure and slow-cooling structure, or between the longitudinal direction and sectional direction of the film.
That is, film design of the phase change optical recording medium includes optical design and thermal design. The optical design requires the optical characteristics of each thin film. The thermal design requires the thermo-physical properties including, e.g., the melting point, melt latent heat, and crystallization temperature of each thin film. The optical constant of a thin film can be measured by using a device such as an ellipsometer. However, it is impossible to systematically measure the thermo-physical properties of a thin film on the nanometer order while removing the effects of other factors, although several researches suggested that they are different from bulk thermo-physical properties. Therefore, experimental parameters are needed to correct these properties. In particular, there is almost no method of measuring the boundary thermal resistance between thin films on the nanometer order. The inventors made extensive studies on these problems as well, and have established a thermal design method in which the thermo-physical properties of thin films and the boundary thermal resistance between them measured by a highly precise method are taken into consideration by thermal design, thereby completing the invention.
(Interface Layer Materials)
As a known technique which can be another interface layer material having a crystallization promoting function differing from GeN, there is a technique which mixes a carbide or nitride in several oxides such as Ta2O5 to form a sulfur (S)-free protective film material (Jpn. Pat. Appln. KOKAI Publication No. 2003-006794). This invention disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-006794 has its main purpose to improve the current DVD using a laser diode (LD) having wavelength λ=650 nm. Therefore, the material of Jpn. Pat. Appln. KOKAI Publication No. 2003-006794 becomes opaque at wavelength λ=405 nm of the next-generation blue LD. Since the optical loss increases, this material cannot be used in the next-generation, high-density medium. GeN described previously is not also transparent at wavelength λ=405 nm, and increases the optical loss.
As a known technique of an interface layer material containing ZrO2, there is a technique related to (ZrO2)M(Cr2O3)100-M, i.e., a Zr—Cr—O series (Jpn. Pat. Appln. KOKAI Publication No. 2003-323743). Cr2O3 mixed in the material series of Jpn. at. Appln. KOKAI Publication No. 2003-323743 is a material having a very large extinction coefficient in the wavelength region of visible light. Even when the amount of material is small, therefore, a thin film having a relatively large extinction coefficient forms if the material is used as the material mixture contained in the film. As described above, none of the presently known techniques can provide an interface layer material which is optically transparent at a wavelength of 405 nm or less and has the crystallization promoting function.
(Material Series of Recording Film)
A eutectic recording film uses the melt erase mode in the erase process as described earlier, so a cap layer need not have the crystallization promoting function. Therefore, details such as the film material and micro-structure have not been studied. In addition, when the eutectic series is used, it is very difficult to perform so-called land-groove recording by which information is recorded on and played back from both a land (L) and groove (G), since the melt erase mode is used as described above. This makes the eutectic series very disadvantageous in increasing the recording density. By contrast, a so-called pseudo-binary recording film material such as Ge2Sb2Te5 has a performance capable of rapidly changing the phase from an amorphous state to a crystalline state in a solid-phase state (a solid phase erase mode). If the recording film is thin, however, the time needed for crystallization becomes relatively long, so it is indispensable to use an interface layer material having the crystallization promoting function. This can also realize land-groove recording.
That is, the data erase process when the eutectic recording film material is used and that when the pseudo-binary recording film material is used are entirely different as phenomena. Accordingly, characteristics required of a cap layer are different from functions such as the crystallization promoting function required of an interface layer. From the foregoing, it is naturally necessary to select an appropriate film material to provide a suitable interface layer material, but details such as the micro-structure and composition have to be further studied.